Polymerizable liquid crystals known as reactive mesogens (RM's) are widely known for their applications in viewing angle compensation films and thin film retarders in liquid crystal display (LCD) applications.
Recently such RMs have been suggested for the use in a patterned retarder in commercially available 3D LCD displays. In these applications, the birefringent nature of the RM is used to alternate the polarization of light emitted from the front of the LCD.
For example in US2009073559 (A1) or WO2011078989 A1, it has also been reported that glass free autostereoscopic displays can be realized by combining an active LC panel as a polarization switch and a polarization sensitive lens made on the front of LCD's.
Such devices comprise an LCD panel for example of the active matrix type that acts as a spatial light modulator to produce the display image. The display panel has an orthogonal array of display pixels arranged in rows and columns. In practice, the display panel comprises about one thousand rows and several thousand columns of display pixels.
The structure of the liquid crystal display panel is entirely conventional. In particular, the panel comprises a pair of spaced transparent glass substrates, between which for example an aligned twisted nematic or another liquid crystal medium is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarizing layers are also provided on the outer surfaces of the substrates.
Each display pixel comprises opposing electrodes on the substrates, with the intervening liquid crystal medium there between. The shape and layout of the display pixels are determined by the shape and layout of the electrodes. The display pixels are regularly spaced from one another by gaps. Each display pixel is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display image by providing addressing signals to the switching elements, and those skilled in the art will know suitable addressing schemes.
The display panel is illuminated by a light source comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source is directed through the display panel, with the individual display pixels being driven to modulate the light and produce the display image.
The display device of prior art comprises a lenticular sheet, arranged over the front side of the display panel, which performs a view forming function. The lenticular sheet comprises a row of lens elements extending parallel to one another. The lens elements are in a form of plano-convex lenses, and they act as a light output directing means to provide different images, or views, from the display panel to the eyes of a user positioned in front of the display device.
In this connection, US 2007/109400 A1 discloses a birefringent lens structure comprising a birefringent lens array capable of directing light of a given polarization into a directional distribution, the birefringent lens comprises a solid birefringent material and an isotropic material having an interface having a refractive structure. A switchable liquid crystal layer capable of rotating the polarization of light passing there through is arranged adjacent the first birefringent material. The interface between the birefringent material and the liquid crystal layer has an alignment microstructure providing alignment of the birefringent material and the liquid crystal layer. A pair of electrodes for applying an electric field to switch the liquid crystal is arranged with both the lens array and the switchable liquid crystal layer there between and a conductive material is incorporated in the lens array to reduce the voltage drop across the lens array. To reduce reflection, the interface between the birefringent material and the isotropic material has an interface having alignment microstructure providing alignment of the birefringent material, and the refractive index of the isotropic material is substantially equal to the extraordinary refractive index of the birefringent material.
JP2012-137616 A1 discloses a birefringent lens material for a stereoscopic image display which contains a reactive mesogenic compound having at least one or more polymerizable functional group, and a non-liquid crystal compound having at least one or more polymerizable functional group, and a manufacturing method of a birefringent lens for a stereoscopic image display using the birefringent lens material for the stereoscopic image display.
However, the use of non-mesogenic compounds or non-liquid crystalline materials in the lens material leads amongst other disadvantageously changes in the optical characteristics, to a drop of the birefringence.
If the birefringence of a suitable RM mixture is increased, the higher birefringence allows a higher radius of curvature (thinner lens) to be used for a given focal length. This phenomenon is well known to the skilled person and for example described in Hecht, E. (2002) Optics, 4th edn., “Geometrical optics” Chapter 5, page 158 et seqq.
In fact, modern applications require suitable high values for the birefringence in order to reduce the lens thickness and therefore the amount of materials and connected costs required to make such a lens. Moreover, the mixtures and/or the resulting lenses have to fulfil beside suitable values for the birefringence a number of other requirements, which are amongst others,                a good room temperature stability against crystallization before polymerization,        an homogeneous planar alignment throughout the whole lens,        high clearing points,        suitable low yellowness index        a good stability against thermal stress, e.g. heat or cold,        a good stability against UV-light,        a good durability in an environment where it is externally exposed,        a good transmission for VIS-light, and        the method of its production has to be cost efficient and suitable for a mass production process.        
In view of the prior art and the above-mentioned requirements on such materials, there is a considerable demand for new or alternative materials, which preferably do not show the drawbacks of the RM materials or mixtures of prior art or even if do so, to a less extend.
Surprisingly, the inventors have found that a birefringent RM lens as described and claimed hereinafter represents an excellent alternative to known birefringent RM lenses, which preferably improves one or more of the above-mentioned requirements or even more preferably fulfils all above-mentioned requirements at the same time.